The lithium-sulfur (Li—S) battery has recently attracted attention due to its potential to meet the performance requirements for high-energy-density batteries in emerging electronics and vehicle applications. Sulfur is a naturally abundant element, nontoxic, and one of the cheapest energy storage materials available, with an extremely high capacity of about 1675 mAh/g. In a Li—S cell, sulfur is electrochemically reduced to polysulfide intermediates through a multistep process, in which longer chain polysulfides present in sulfur tend to dissolve in organic electrolytes commonly used in lithium battery applications, which is undesirable. Insoluble discharge products, such as Li2S2 and Li2S, also are generated through the reduction reactions at the final step. During the charging step, Li2S/Li2S2 is converted to elemental sulfur through the multiple oxidation steps.
The dissolution of the intermediate lithium polysulfides during cycling causes a severe redox shuttling effect and rapid capacity fading, which are the main obstacles for commercialization of Li—S batteries. A firm understanding of the operation mechanism of the Li—S battery and the technical solution to solve these issues are in great demand in order to successfully develop Li—S batteries for commercial application. Much research has been undertaken to overcome these problems. One approach was to introduce porous carbon materials into the cathode to trap lithium polysulfides within the cathode during cycling by the strong adsorption property of carbon. Another approach was to form a protective layer on the lithium anode surface to mitigate the redox reaction of the dissolved polysulfides and lithium metal. Yet another approach was the development of new solid state electrolytes including ionic liquids, tetra(ethylene glycol) dimethyl ether as organic solvents for the electrolyte, lithium salt electrolytes, and functional electrolyte additives to prevent the dissolution of the polysulfides into the organic electrolyte and thereby avoid the redox shuttling effect. While these approaches can improve the Li—S performance to some extent, there is an ongoing need for new electrolyte compositions for Li—S batteries. The compositions described herein address this need,